The present invention relates to a method of controlling load on a thermal power plant and, more particularly, to a method of controlling load on a power plant in which the flow rate of fuel supplied to a gas turbine is regulated to control load on the power plant.
A thermal power plant controls the flow rate of fuel supplied to a gas turbine installed therein or the flow rate of steam supplied to a steam turbine installed therein according to a load dispatching instruction given to the thermal power plant by a central load dispatching station. The system frequency (50 Hz in the Eastern Japan and 60 Hz in the Western Japan) is stabilized by adjusting power generation through the adjustment of shaft speed by a speed signal according to contribution ratio also called speed droop ratio. For example, the flow rate of fuel supplied to the combustor of a gas turbine is controlled on the basis of system contribution ratio according to information about system frequency varying according to the variation of total load on the system. This mode of control is called governor load control or governor-free control.
For example, when the 100% speed of a power generator of 5% in speed droop ratio is 50 Hz, governor load control adds fuel for 100% rated power generation capacity per 2.5 Hz equal to 5% speed to an initial fuel supply rate. Actual system frequency is relatively stable during operation at a rated output power and hence frequency change is 1% or below.
As shown in FIG. 10, the relation between system frequency and fuel supply rate in governor load control adds 20% of fuel supply rate per 1% to an initial specified flow rate or subtracts the same from the initial specified flow rate because 100% load adjustment per 5% is necessary if frequency change is 1%. The function of governor load control stabilizes system frequency and reduces fuel supply rate when the rotating speed of a turbine increases sharply due to power system fault to prevent the operation of a turbine or a power generator at an excessively high operating speed. Since a plant of 5% in speed droop ratio operates fuel supply rate for 100% load for 5% according to rotating speed, a load output instruction becomes 0% instantaneously when overspeed rises to 105%, and the output becomes negative, i.e., rotation is braked, when shaft speed exceeds 105%.
In a gas turbine power plant, fuel and compressed air is supplied to a combustor, and a gas turbine is driven by a combustion gas produced by the combustor. When governor load control is performed, fuel supply rate is adjusted at a high gain as mentioned above for the slight change of system frequency. When the fuel supply rate instruction changes sharply due to high gain or frequently, the fuel-air ratio of the mixture supplied to the combustor changes suddenly causing unstable combustion, misfire or backfire. If fuel supply rate increases to cause temperature to rise beyond an upper limit or to induce thermal stress, thermal stress in the gas turbine increases and thermal stress is accumulated in the gas turbine. The thermal stress is a principal cause of the deterioration of mechanical parts, such as the combustor and turbine blades and shortens the life of the equipment.
A gas turbine control system proposed in Japanese Patent Laid-open No. Hei 8-218897 to solve such problems performs, sets an upper limit to the temperature of the exhaust gas of a gas turbine, adds an offset to a fuel supply rate instruction for governor load control, gives temperature load control priority when the temperature difference between the upper limit and the temperature of the exhaust gas approaches naught to maintain the load on the gas turbine constant. Thus, the gas turbine control system prevents induction of an excessively high thermal stress in the gas turbine.
In conventional thermal power plants, control outputs of stop control, acceleration limiting control and load limiter, in addition to those of governor load control and temperature load control, are used in combination, and a control signal which takes a low value according to the pattern of operation and the condition of the plant is selected to control the load on the gas turbine.
However, combustion gas temperature rises near to the upper limit when the gas turbine operates under a load nearly equal to its rated load, temperature load control is selected and governor load control does not function if the control system of the cited reference is employed. Therefore, a speed droop ratio set on the basis of the power generation plan of the power system cannot be maintained. When the gas turbine operates in a partial-load operating mode in which temperature load control is not selected, governor-free control is executed. However, problems arising from unstable combustion due to the change of control instruction cannot be solved and many problems reside particularly in equipment requiring difficult fuel-air ratio control, such as a low-NOx combustor. Thermal stress is induced concentratedly on turbine blades. The same problems reside in steam turbines in which steam flow rate varies.
Thus, in the conventional thermal power plants, frequency change due to the change of load on the power system cannot be suppressed if temperature load control is performed, regarding the life shortening effect of thermal stress and unstable combustion as important. When governor load control is performed to maintain speed droop ratio when the power system is unstable, thermal stress and unstable combustion is enhanced because load instruction varies in a wide range when the power system is unstable. Thus, there are conflicting problems.
It is an object of the present invention to solve those problems in the prior art and to provide a method of controlling load on a thermal power plant, capable of maintaining system contribution by governor load control, of extending the life of equipment and of stabilizing combustion.
Another object of the present invention is to provide a load control system for controlling load on a thermal power plant, capable of maintaining system contribution by governor load control, of extending the life of equipment and of stabilizing combustion.